1. Field
The present invention relates to the detection of electric fields, and more particularly, to apparatus and methods of detecting stray voltage anomalies in electric fields.
2. Description of the Related Art
Large power distribution systems, especially those in large metropolitan areas, are subject to many stresses, which may occasionally result in the generation of undesirable or dangerous anomalies. An infrequent, but recurrent problem in power distribution infrastructures is the presence of “stray voltages” in the system. These stray voltages may present themselves when objects, such as manhole covers, gratings, street light poles, phone booths and the like become electrically energized (e.g., at 120V AC). An electrically conductive path may be established between underground secondary network cabling and these objects through physical damage to electrical insulation resulting in direct contact between electrically conductive elements or through the introduction of water acting as a conductor. These energized objects present obvious dangers to people and animals in the general public.
Detecting the existence of stray voltages by means of assessing electromagnetic radiation is not practical because the wavelength of a 60 Hz electromagnetic wave is approximately 5,000 kilometers (i.e., about 3,107 miles) in length. To effectively radiate electromagnetic waves, a radiating object (e.g., manhole cover or light pole) should represent at least ¼ wavelength (i.e., about 776.75 miles) and a receiving “antenna” should be 1½ to 2 wavelengths away from the emitting source (about 6,214 miles). Two wavelengths is the distance required for electric and magnetic fields to come into time phase and space quadrature where they behave as a plane wave. A detection system will typically be perhaps 10 ft. to 30 ft. away from the energized object, so that detection will take place in the extreme near field where electric and magnetic fields exist in a complex temporal and spatial pattern, not as a unified electromagnetic plane wave. Thus, electric and magnetic fields must be considered and measured separately.
Due to power distribution networks typically having many miles of buried cable carrying perhaps thousands of amperes of current, the magnetic field in any one location due to such normal load is likely to be very high. Detecting magnetic fields arising from a relatively weak stray voltage anomaly would be very difficult due to the interference from strong ambient magnetic fields arising from normal loads and, therefore, it has been determined that the best way to detect a stray voltage anomaly is to assess the electric field.
Techniques for the detection of stray voltages are typically carried out by manual inspection of surrounding electrical infrastructures for signs of leaking current. An inspection team equipped, for example, with hand held detection devices may be employed to make direct physical inspections of electrical infrastructures. However, inspectors using these detection devices are typically required to make contact with portions of electrical infrastructures, such as streetlamp bases or manhole covers, in order to obtain accurate measurements for determining the existence of potentially dangerous stray voltages. These manual inspections are undoubtedly time-consuming and give a false sense of security.
Accordingly, there exists a need to provide a more efficient means for detecting and identifying sources of stray voltage anomalies over vast geographic areas, particularly, populated urban, suburban and rural areas.